Nematodes are small, worm-like animals frequently associated with a variety of destructive diseases in plants, animals, and humans. Pathogenic nematodes are a major agricultural problem, causing significant crop and yield losses worldwide. The potato cyst nematodes Globodera rostochiensis and Globodera pallida, for example, are key pests of the potato, while the beet cyst nematode Heterodera schachtii is a major problem for sugar beet growers in Europe and the United States. Furthermore, losses due to these nematodes have enormous economic impact. For example, losses due to the soybean cyst nematode (SCN) Heterodera glycines may exceed $500 million/year in the United States alone, or about 10% of the total $5.8 billion/year of agricultural losses due to nematode destruction of crops in the United States.
Because of the large costs of nematode-related agricultural losses, a variety of methods for controlling nematodes in plants have been developed, primarily involving the use of highly toxic chemical nematicides. Thus, although naturally nematode-resistant varieties of crop plants can be used to reduce the effects of nematodes on crops, in general, control has focused on the use of highly potent carbamate chemicals such as Aldicarb™ (Rhone-Poulenc Ag. Co). In 1982, in the United States alone over 100 million pounds of such nematicides were applied to crops. However, the use of chemical nematicides has been restricted in recent years because such compounds have been shown to be highly toxic to mammals. Thus, there is a real need for effective non-chemical nematicides and methods for the treatment of nematode diseases in plants.
One such method is the production of transgenic plants that are resistant to nematode infections. For example, with the use of nematode-inducible promoters, plants can be genetically altered to express nematicidal proteins in response to exposure to nematodes. See, for example, U.S. Pat. No. 6,252,138, herein incorporated by reference. Alternatively, some methods use a combination of both nematode-inducible and nematode-repressible promoters to obtain nematode resistance. Thus, WO 92/21757, herein incorporated by reference, discusses the use of a two-promoter system for disrupting nematode feeding sites, where one nematode-inducible promoter drives expression of a toxic product that kills the plant cells at the feeding site, while the other nematode-repressible promoter drives expression of a gene product that inactivates the toxic product of the first promoter under circumstances in which nematodes are not present, thereby allowing for tighter control of the deleterious effects of the toxic product on plant tissue.
Although these methods have potential for the treatment of nematode infections, their effectiveness is heavily dependent upon the characteristics of the nematode-inducible or nematode-repressible promoters discussed above. Thus, such factors as the strength of such nematode-responsive promoters, degree of induction or repression, tissue specificity, or the like can all alter the effectiveness of these disease resistance methods. Consequently, there is a continued need for the identification of nematode-regulated promoters for use in promoting nematode resistance.